Pterosaur ontogeny

Not too long ago, Matt Wedel had an SV-POW! post that talked about ways of diagnosing an adult vs non-adult sauropod. Inspired by this and the fact that I have recently been playing around with issues of ontogeny in pterosaurs, I decided to write something similar for the non-avian Mesozoic fliers. If you have a pterosaur specimen in front of you, just how do you know if it’s an adult or not?

Obviously there are some general indicators that are pretty good for vertebrates as a whole that will get you quite a long way (even if this is a new species). Size is obviously rarely a great indicator, but if you have a pterodactyloid with a 20 cm wingspan then it’s going to be a juvenile, and likewise if you have a rhamphorhynchoid coming in close to the 2 m mark it’s very unlikely to be anything but a big adult. Young animals (and especially very young animals) tend to have big heads compared to their body and especially very big eyes compared to the size of the head. A bunch of fusions are absent in young pterosaurs that are present in adults too, just as you’d expect for most animals. The sutures between the centrum and neural arch of the vertebrae will be open in juveniles and closed in adults, and similarly the elements of the pelvisand sacrum, and the scapula and coracoid will be separate in young animals and fused together in adults.

Pterosaurs also have some characters of ontogenetic change that are rather more peculiar to them than vertebrates in general. Very young pterosaurs also tend to have a very grainy texture to the surfaces of their longbones, despite the fact that even embryonic pterosaurs have a pretty ossified set of bones (unlike many young animals). Smaller pterosaurs also tend to have various parts of the skeleton being less ossified and rather amorphous compared to those of adults. The tarsals are often not well ossified and can be missing (well don’t preserve) and if present may be very simple shapes. The carpals tend to look more ‘blobby’ and lack the detailed morphology seen in adults and will be separated into multiple elements whereas in adults the wrist will primarily be formed of just two massive elements (plus the pteroid). Finally, while obviously you would expect skulls to fuse up during ontogeny, pterosaurs do tend to take it one step further than most. Rather like birds, in adult pterosaurs the sutures all but disappear, or even go entirely, such that the skull looks like a single smooth piece of bone. Also as in some birds, bigger pterodactyloids have a notarium and this only fuses up and fully develops in adults. Similar to the point above about absolute size, the presence and development of some form of head crest is indicative, but not a great indicator of age. Yes a massive and elaborate crest in an animal is indicative that it’s an adult, but there could be a fairly well developed crest in an animal that is close to becoming and adult and of course there are taxa without crests and in at least once case it appears that females don’t have crests.

As in mammals, but unlike dinosaurs and birds, pterosaur also have epiphyses. The growing plates at the ends of the long bones physically separate the main shaft of the bone from the proximal and distal ends, so things like the femur can appear to be in three pieces. Obviously as growth slows towards maturity these epiphyses slowly disappear as they fuse into the single element that you would expect to see.

So in short, something that is small, with grainy textured bones, a big head, with big eyes, unossified tarsals, amorphous carpals, no crest, clear sutures in the skull, no notarium, and separated scapulocoracids, pelvis, epiphyses and neurocentral sutures is going to be a young juvenile. And the close these various features get to the opposite condition the closer the animal is likely to be to adulthood.

As ever with such things these are not absolutes, but merely guides. Good guides, certainly – you simply won’t see a notarium in a very young pterosaur, or open neurocentral arches in a big, old adult. However, in terms of determining more subtle difference in age it will be tricky – one animal may have fused up the notarium, but may have incompletely ossified tarsals and another could have the reverse. Although at least some characters do seem to have a bit of a pattern (the scapulocoracoid seems to fuse pretty early in most things) a general lack of numerous specimens of different ages makes it hard to do any more detailed analysis. Still, in terms of gross age (hatchling – young – adolescent – adult) even for a specimen of a previously unknown species with no obvious close relatives, it should be relatively easy to determine the approximate age of the animal.

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8 Responses to “Pterosaur ontogeny”

Interesting. I assume this is because of the functional demands of flight. I am intrigued because you’d think that sauropod limb girdles would be under functional demands for weight-bearing and would also fuse up early, but the scap-coracoid joint is one of the last things to fuse in the postcranial skeletons of sauropods. So clearly my gut instinct about how this stuff ought to work is wrong.

Someone ought to survey archosaurs in general looking at the relative timing of these post-hatching developmental fusions.

“Someone ought to survey archosaurs in general looking at the relative timing of these post-hatching developmental fusions.”

Indeed! I’ll get onto it as soon as I’ve got all my other work done. And all the taxonomy vs ontogeny issues for all archosaurs are sorted. 😉

As for the actual point, agreed – flight seems the obvious issue for pterosaurs, but weightbearing not in sauropods? But then I suppose that weigh is directed in a fairly consistent manner, and remember that the forelimbs are taking pretty much all the power when pterosaurs fly and most of the weight when on the ground (and indeed launching) when sauropods are more back heavy (even if the total mass is still high). As you say, needs more thought and research, but would be most interesting to look at.

We know quite a lot, but it’s rather spread out (i.e. there’s no particular paper that unifies and sysnthesises all of this – oddly enough I’ve more or less suggests this as a grant proposal a couple of times and got nowhere with it which is annoying). So we do know that pretty much all of them were penumatic, and that this probably increased with ontogeny and phylogenetically (i.e. on average older ones had more pneumaticirty than young individuals and that more derived ones were more pneumatic than more basal ones). After that, not much. Various details for sure, but nothing like a massive review and analysis.

Short version is that pterosaurs are most probably the sister-taxon to the dinosauromorphs (i.e. the dinosaurs and their immediate ancestors). It’s not really as certain as say the dinosaur-bird link because we really do lack transitional fossils here and are limited in the number of characters and shared anatomical characters that we can bring to the party, but this position is much better supported than anything else. There are some other issues but this is the big one.

This also kinda answers your second question since we don’t have things like Microraptor or Archaopteryx when it comes to pterosaurs. I think the general assumption is that there was probably a gliding ancestor, but there’s no direct evidence. All the pterosaurs we have were fully capable of powered flight so quite what the pre-flying ones were doing is hard to know.